The present invention relates to extreme ultraviolet lithography (EUVL). More particularly, it relates to EUVL apparatus made of silica-based light-weight material.
Extreme ultraviolet (EUV) lithography (EUVL) is a relatively new form of lithography. EUVL uses extreme ultraviolet (EUV, also called soft X-ray) radiation with a wavelength in the range of 10 to 14 nanometers (nm) to perform the imaging. Up to now, optical lithography has been the lithographic technique of choice in the high-volume manufacture of integrated circuits (IC). The constant improvement in optical lithography has allowed it to remain the semiconductor industry""s workhorse through the 100 nm or smaller generation of devices. However, to pack even higher density circuits into IC chips, new technologies (Next-Generation Lithographies, NGL) will be required. EUVL is one NGL technology vying to become the successor to optical lithography.
In many respects, EUVL is similar to the optical lithography. For example, as illustrated in FIG. 1, the basic optical design for an EUVL system is similar to that of an optical lithographic system. It comprises a light source 1 of light 12, a condenser 2, a mask (reticle) 4 on a mask stage 5, an optical system 6, and a wafer 7 on a wafer stage 8. Both EUV and optical lithographies use optical systems (cameras) to project images on the masks onto substrates which comprise silicon wafers coated with photo resists. However, the apparent similarity stops here. Because EUV is strongly absorbed by virtually all materials, EUV imaging must be performed in vacuum, which is achieved by enclosing the system in a chamber 3. In addition, the chamber 3 might be further partitioned into different compartments 10 and 20, which have their own vacuum systems. Because EUV is absorbed by most materials, there are no suitable lenses to focus and project an image on mask 4 onto a substrate (wafer) 7. As a result, it is necessary to operate EUVL in a reflective mode instead of a transmissive mode. In the reflective mode, light is reflected from mirrors (not shown; inside the optical system 6), instead of being transmitted through lenses. Even with reflective optics, there are not many materials capable of reflecting EUV. In order to achieve reasonable reflectivities at near normal incidence (i.e., an incident beam landing on the surface of a mirror at an angle close to normal to the surface), the surface of a mirror is typically coated with multilayer, thin-film coatings. These multilayer, thin-film coatings reflect EUV in a phenomenon known as distributed Bragg reflection.
The multilayer coatings for the reflective surfaces in EUVL imaging system consist of a large number of alternating layers of materials having dissimilar EUV optical constants. These multilayers provide a resonant reflectivity when the period of the layers is approximately xcex/2, where xcex is the wavelength. The most promising EUV multilayers are coatings of alternating layers of molybdenum (Mo) and silicon (Si). These layers are deposited with magnetron sputtering. Each layer of Mo or Si is coated to a thickness of xcex/4 of the EUV light so that it will have a periodicity of xcex/2. In this type of reflector, a small portion of the incident light is reflected from each silicon surface. The thickness of the layers causes the reflected light waves to interfere constructively. The more layers there are, the more light will be reflected. However, imperfections in the surface coating will eventually diminish the reflectivity return of more coatings. Currently, most mirrors in EUVL systems have around 40 alternating layer pairs of Mo:Si. Furthermore, most of these Mo:Si multilayers are optimized to function best with wavelengths at around 13.4 nm, which is the wavelength of a typical laser plasma light source.
Like the mirrors, the masks for EUVL application are reflective, not transmissive, because little EUV light would transmit through a mask. The reflective masks are manufactured of Mo:Si multilayers, much like the mirrors. However, the magnetron sputtering techniques used for the manufacturing of mirrors are not suitable for the masks because the defect rates with magnetron sputtering are too high to be used in making the masks. Instead, ion-beam sputtering, which has been shown to achieve several orders of magnitude less density of defects, is used to make the masks.
In the operation of an EUVL system, EUV light 24 is reflected off mask 4. The reflected light 47 is further reflected by mirrors (not shown) inside the optical system 6, which is used to direct and reduce the dimension of the reflected light 47. The reduction in dimension achieved by the optical system 6 allows a smaller image (than that on the mask 4) to be printed on wafer 7. Typically, such a printing process is performed in a step-and-scan mode, in which both mask 4 and wafer 7 need be moved in a precise and coordinated fashion. Such movement is made possible by placing mask (reticle) 4 and wafer 7 on movable stages 5 and 8, respectively, which are controlled by a computer (not shown). Stage 5 or 8 typically comprises an X-Y platen (a platen that can move in the X and Y directions) with a holder (not shown) to hold the mask 4 or the wafer 7. Movement of the stage is accomplished with motorized positioning devices that are controlled by a computer. These stages also facilitate the alignment of the components in the initial system setup.
In a projection lithographic system, precise alignment of the components (e.g., the mask, the mirrors, and the wafer) is critical for the production of an accurate pattern on the wafer substrate. The precision required for EUVL is more stringent than for optical lithography. These factors motivated the development of high-accuracy EUV wavefront measuring interferometry to determine the performance of an EUVL system at the operating wavelength or to align and adapt the system to the desired characteristics. For these purposes, easily adjustable components are essential. Having each component on an adjustable stage will facilitate such adjustment.
Whether during an alignment or scanning operation, the heavy mass of the stages impedes the accuracy and performance because the heavy mass requires more powerful motors which generate more heat. Heat causes the system temperature to rise and an EUVL system is highly sensitive to temperature changes. In addition, heavier mass hampers maneuverability of the system (i.e., difficult to achieve high precision control in the scanning operation). Therefore, in order to have easy maneuverability, hence improved positioning and scanning accuracy, it is desirable to have these components made of light-weight, stiff materials, or to use magnetic levitation to reduce the weights of the components. While magnetic levitation has been successfully used in the early development of EUVL systems, attention has now turned to the use of light-weight materials.
The use of light-weight materials for the construction of mirrors and optical components system components is not new, at least in the construction of mirrors. For example, Mary J. Edwards et al. describe methods of making light-weight mirrors in, xe2x80x9cCurrent Fabrication Techniques for ULE(trademark) and Fused Silica Light-weight Mirrors,xe2x80x9d V 3356, pp. 702-11, Proceedings of SPIE, Mar. 25-28, 1998. The methods involve machining out solid pieces of glass to produce a light-weight structure or fusing pieces of glass into a light-weight core. More recently, methods for making light-weight stages have also been reported. For example, Hrdina et al. in U.S. application Ser. No. 09/502,251, filed on Feb. 17, 2000, U.S. application Ser. No. 09/506,040, filed on Feb. 17, 2000 U.S. application Ser. No. 09/506,162, filed on Feb. 17, 2000, and U.S. Pat. No. 6,118,150, issued to Spence, disclosed a light-weight high stiffness stage platen. The platen disclosed in the Spence ""150 patent was constructed of a thin plate and reinforced with a matrix of rib structures to give it the required stiffness.
In addition to minimizing the weight of components, an ideal EUVL system should also have good thermal response. That is, the system should not be sensitive to temperature changes. Therefore, it is desirable to have light-weight, low thermal expansion stages for use in EUVL systems.
One aspect of the invention relates to light-weight stages for use in EUVL systems. One embodiment comprises a platen having a top face and a bottom face and a holder for holding an optical component on the top face of the platen. The platen may comprise a light-weight material such as high purity fused silica or ultra low expansion glass. In other embodiments, the bottom face of the platen may further comprise means for connecting to a positioning device in an extreme ultraviolet lithography system.
Another aspect of the invention relates to methods for manufacturing light-weight EUVL stages. In one embodiment, a method for manufacturing an extreme ultraviolet lithography stage comprises extruding an included void structure from a glass material, and shaping the included void structure into the extreme ultraviolet lithography stage. The shaping may comprise fusing a thin piece of glass material onto the included void structure
Another aspect of the invention relates to EUVL systems. In one embodiment, a lithographic projection system comprises a radiation system to supply a projection beam of radiation; a mask stage holding a mask, the mask stage being made of a light-weight material; a substrate stage holding a wafer, the substrate stage being made of the light-weight material; and an optic system for imaging an irradiated portion of the mask onto a target portion of the wafer. In another aspect, the glass material is preferably a TiO2 containing silica glass, preferably a fused silica ultra low expansion glass, preferably with a TiO2 content in the range of to 10 wt. % TiO2, more preferably 6 to 8%, most preferably 6.5 to 7.5%.
Other aspects and advantages of the invention will be apparent from the accompanying descriptions and drawings.